The present invention relates to an apparatus for feeding synthetic resin powder to a screw extruder, and, particularly, to an apparatus for feeding synthetic resin powder to a screw extruder, in which a vertical pipe-line having an orifice is provided between a vacuum hopper directly connected to the screw extruder and a raw synthetic resin powder tank, so that the raw synthetic resin powder may be continuously for intermittently fed into the vacuum hopper while the degree of vacuum in the hopper is maintained substantially constant.
In producing various kinds of products by using a screw extruder, a method has been generally used in which a raw material polymer is fed to the screw extruder after being made into the form of a pellet. Another method is also employed, however, in which, when the raw material polymer is in the form of powder, it is directly fed to the screw extruder as it is. The latter method is preferably employed in the case where the raw material is a synthetic resin powder which does not respond well to an increase in its thermal history.
There is a disadvantage in the latter method, in that if air enters the screw extruder together with the synthetic resin powder, not only may the quality of the product be deteriorated, but the raw material synthetic resin may be easily decomposed, so that the decomposed material adheres to the inside of the screw extruder. This necessitates frequent cleaning of the insides of the screw extruder, resulting in lowered production efficiency.
Thus, it is considered that when synthetic resin powder is applied to a screw extruder a hopper directly connected to the screw extruder is maintained at a vacuum to prevent air from entering the inside of the screw extruder and at the same time to discharge gases produced in the screw extruder.
To this end, an apparatus as shown in FIG. 1 has been conventionally used. That is, two hoppers B and C are provided above a screw extruder E, and synthetic resin powder is fed to the hoppers from a raw material tank A using a vacuum pump D. The feeding of the synthetic resin is performed such that the upper stage hopper B is first made vacuous so as to vacuum-attract the raw synthetic resin powder, which is then transferred to the lower hopper C, and then again evacuated while the raw synthetic resin powder is continuously fed into the screw extruder from the lower hopper C. To accomplish this end, it is necessary to alternatingly open/close numbers of valves.
In such a machine, the operation is complex and a complex controller is therefore required. Further, it takes a long time (usually, about eight hours by two persons) to reassemble the vacuum hopper after overhaul cleaning of the hopper has been performed for the purpose of preventing foreign raw materials from being mixed when the type of raw synthetic resin is changed. This is because accuracy is required in assembling the hopper since it is necessary to maintain the inside of the hopper at a high degree of vacuum when the vacuum line is closed in the operation of the screw extruder.